Comparison of Various Assays of Antioxidant Activity/Capacity: Limited Significance of Redox Potentials of Oxidants/Indicators

Assays of total antioxidant capacity (TAC) of complex Materials bring no information on the composition of Antioxidants present in a sample. As the thermodynamic condition for a redox reaction is that redox potential of the oxidant must be higher than that of a reductant (Antioxidants), it seemed to be of interest whether it is possible to estimate the content of Antioxidants of various ranges of redox potentials using a set of assays employing oxidants/indicators of different values of redox potentials. Antioxidant activities of nine Antioxidants and TAC of an aqueous garlic extract were estimated using nine assays of E_o' of oxidants/indicators ranging from 0.11 to 1.15 V. The antioxidant activities were expressed in mol Trolox equivalents/mol compound. The thermodynamic conditions made some Antioxidants unreactive with indicators of sufficiently low E_o', but otherwise, no dependence between the antioxidant activities and redox potentials of oxidants/indicators and reactivities of Antioxidants was observed. TAC of the garlic extract did not show any regular dependence on the redox potential of the oxidant/indicator, being the highest in the test of 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonate) radical (ABTS^•) decolorization. These results indicate that kinetic factors play a primary role in determining the antioxidant activities of Antioxidants and TAC in various assays.

2,6-dichlorophenolindophenol; ABTS; Cupric Ion Reducing Antioxidant Capacity; Ferric Reducing Antioxidant Power; Oxygen Radical Absorbance Capacity; TAC; Trolox; antioxidants; ferricyanide; o-phenanthroline.